Sir Alex Ferguson has accepted responsibility for Manchester United's third missed penalty of the season and insisted that he will decide who takes the next spot kick after Nani became the latest player to fluff his lines from 12 yards.

Nani's poor effort could easily have proved costly on a night when United failed to build on Michael Carrick's early goal and endured several nervous moments in a 1-0 win against an impressive Galatasaray side.

Ferguson, who become the first manager to win 100 Champions League games, accused his players of being "very careless" and admitted United looked vulnerable whenever Galatasaray broke forward. The Turkish champions should also have been awarded a penalty inside the first minute, when Nemanja Vidic clearly brought down Umut Bulut, only for Wolfgang Stark, the German referee, to wave away their appeals.

It was, however, the penalty that United squandered which concerned Ferguson most. Robin van Persie and Javier Hernández had missed against Southampton and Wigan respectively but Ferguson still expected the Dutchman to step forward here. Instead Nani seized the ball and saw his weak kick easily saved by Fernando Muslera. United have now missed all three of their penalties this season.

Asked about Nani taking over penalty duties, Ferguson replied: "To be honest with you, I had no idea. I think Nani just ran and grabbed the ball. I thought Robin should have taken it. I think we will have to decide before the game – that was my fault, I should have done that."

Although United's performance was disappointing, Ferguson said winning the first game was crucial in the wake of their failure to progress beyond the group stage last year. "After the experience of last season, which was an unusual one for us, the result was the most important thing," he said. "For our club it is more than that, it is about playing the brand of football we are recognised for and at times we did.

"The first 15 minutes we played very well and got the goal, of course. At that moment we thought it was going to be a good night. But we became very careless and every time we lost the ball, they counterattacked and they were a threat. Fortunately half-time came at a good time for us because we were far more solid [after it]."

Ferguson, who ruled out any prospect of Wayne Rooney returning from injury to feature against Liverpool at Anfield, was delighted to see Darren Fletcher make a late appearance from the substitutes' bench. The Scotland international has been out for 10 months with ulcerative colitis. "I think the supporters recognise what a tough time he has had for the last year and that was a mark of respect. They gave him a marvellous reception," the manager said. "It took him one or two minutes to get going but he can only be better after tonight."